A typical occupant detection system includes a capacitance-type sensor and an electronic control unit (ECU) used for detecting an occupant. An electrode of the capacitance-type sensor is disposed in a seat to form a high frequency electric field around the electrode. When a human body, which is a dielectric material, approaches the electrode, the high frequency electric field is disturbed. The occupant detection ECU detects this disturbance of the electric field as a variation of the capacitance. That is, the occupant detection ECU determines that an occupant is on the seat from the variation in the capacitance. Such a system is disclosed in Japanese Patent Publication No. 7-78539.
Additionally, Japanese Patent Laid-Open Publication No. Hei 8-225058 discloses an occupant detection system utilizing an electrode built into a seat and a radio frequency (RF) tag built into a child seat. This occupant detection system can distinguish between an adult and a child seat.
The electrode of the capacitance-type sensor described in both of the above-identified patent references, however, is disposed substantially over the entire surface of the seat, so the structure of the capacitance-type sensor is complex. Therefore, the manufacturing cost of the capacitance-type sensor and accordingly that of the occupant detection system are high.
Furthermore, the occupant detection system described in the second patent reference can determine between an adult and a child seat only when an RF tag is built into the child seat, thereby lacking versatility.